This invention relates generally to the field of telephone protector modules of a type employed for protecting individual telephone subscriber circuits, and more particularly to an improved carbon element capable of withstanding the effects of heat generated by the transmission therethrough of very high current flow.
As is known in the art, protector modules are usually installed upon a protector block mounted upon a mainframe in a telephone office to prevent the conducting of high current surges through office switching and other equipment, and to prevent damage to the subscriber circuit wiring by conducting such surges to a source of ground potential. Such devices are normally provided with several pairs of carbon electrodes which are in series with the tip and ring circuits of each line, the pairs of carbons defining gaps over which momentary surges of current may arc to be conducted to ground potential. When the surge is of longer duration, as occurs, for example when a high tension line has fallen on the telephone line, heat caused by current flow melts a lead pellet or soldered joint forming part of a heat detecting means. Upon melting, a resiliently actuated component forms a direct contact with ground potential, bypassing the electrodes. Once this actuation has occurred, the individual subscriber line is disabled, and can be returned to operative condition only by replacing the protector module. It is for this reason that the carbon electrodes are provided which will normally withstand many momentary surges without disabling the subscriber circuits.
While this known construction has functioned reasonably well, a problem does exist in that the melting of the solder, depending upon the magnitude of the excess current flow normally requires a period of between fifteen to forty seconds, and during this period, the current will arc through the carbon electrodes to reach ground potential. Current flows of as much as two hundred fifty amperes are common, and the heat developed thereby is often sufficient not only to destroy the module, but other modules positioned upon the same protector block, thereby disrupting service in the related subscriber circuits as well. Given the typical resistance of normal carbon electrodes of approximately 0.1 ohm, the average wattage carried by the carbon prior to the firing of the protective device is approximately four thousand watts, and the heat developed in the carbon is approximately fifteen hundred degrees Fahrenheit reached during the above mentioned fifteen to forty second period. The casing element of the module is of necessity electrically insulated, and the synthetic resinous materials normally used for this purpose are not adequate to withstand such temperatures.
There are any conductive materials having relatively high specific heats which are substantially greater than that of the bonded carbon particles used to form the conventional electrode. Most of these are metallic alloys, and do not offer sufficient resistance to arcing in the case of momentary current surges. Such materials are also subject to rapid erosion during arcing, making them unsuitable for use as electrodes after a relatively few number of current surges.
It will be appreciated that because of the relatively limited space available for mounting a module upon a protector block, and the large number of such blocks in use at the present state of development of the art, extensive redesign of existing protector modules to avoid the above described problem is not a feasible alternative.